1. Field of the Invention
This invention relates to image formation apparatus for forming a color image, having a combining member for overlapping a plurality of different mono-color half tone or continuous tone images formed by an electrophotography process with a plurality of different colorants such as yellow, magenta, cyan, and black to form the color image thereon.
2. Description of the Prior Art
A color copy apparatus is known as a color image formation apparatus for forming a color image, having a combining member for overlapping a plurality of different mono-color half tone or continuous tone images formed by an electrophotography process with a plurality of different colorants such as yellow, magenta, cyan, and black to form the color image thereon. Such a prior art color copy apparatus comprises a photosensitive member, a charger for charging the photosensitive member, an optical system for consecutively forming and exposure each of primary color images and an achromatic color (black) image of half tone or continuous tone onto the charged photosensitive member to obtain electrostatic latent images, a developing system for consecutively developing the primary color images and the achromatic color image on the photosensitive member from the electrostatic latent images, and image combining member for receiving, overlapping, and combining each of primary colors and achromatic color images developed on the photosensitive member consecutively and a transfer unit for transferring the combined primary color and achromatic color images onto a paper for recording.
FIG. 4 is a side view of the prior art color image formation apparatus for showing a general structure which is common to the embodiment of this invention. FIG. 5 is a graph showing a general relation between an input image density and an output image density, that is a .gamma. characteristic, which is common to the embodiment of this invention. FIG. 6 is a block diagram of a prior art density measuring unit for measuring a reflective density of a portion of image formed on the combining member 6. FIG. 7 is a schematic circuit diagram of the prior art density measuring unit shown in FIG. 6. FIG. 8 is a graph showing relations between densities of input images of the primary colors and the achromatic color (black) image and output voltages of the prior art measuring unit 43 shown in FIG. 6.
In FIG. 4, a photosensitive member 1 formed in an endless belt is supported between rollers 2 and 3 with a suitable tension and rotated in the direction shown by an arrow A. An optical system 4 forms consecutively forming primary color images, such as yellow, magenta, and cyan, and an achromatic image, such as a black onto the photosensitive member 1 through electrophotographic process. During formation of the images, the photosensitive member 1 circulates and the optical system 4 scans a surface of the photosensitive member 1 with a laser beam to form a color image by forming each of primary color images and the black image on the photosensitive member 1 in phase with circulation of the photosensitive member 1. A charger 42 charges the photosensitive member 1 prior to exposure of each of primary color and black images. A developing system 5, having yellow, magenta, cyan, and black developers 5y, 5m, 5c and 5b, consecutively develops primary color images of yellow, magenta, cyan, and black image using colorants, such as toners, with correspondence to exposure of respective primary color images by the optical system 4. An image combining member 6 formed in an endless belt supported by roller 8, 9, and 10 with a suitable tension receives, overlapping and combining each of consecutively developed primary and achromatic color images. The image combining member 6 is circulated in the direction shown by an arrow B and its portion is contacted with a portion of the photosensitive member 1 to receive each of primary color and black images from the photosensitive member 6. The image combining member 6 combines primary color and black images by consecutively receiving the developed primary color and black images and transfer the combined color image to a paper 13 by the transferring unit 14.
It is possible to form a color image by combining primary color images, such as yellow, magenta, and cyan. However, generally, in order to improve a picture quality of color image, a black image of an achromatic color is added to the color image which has been formed by combining primary color images.
A density sensor 7 measures a density of a portion of a color image formed on the image combining member 6 by emitting a light and receiving a reflected light from the image combining member 6. The image combining member 6 has a intermediate reflectivity between those of colorants of primary colors and the achromatic color, i.e., black with respect to the emitted light. A position sensor 11 detects a reference marker, for example a hole provided to the image combining member 6 for generating position signals of the image combining member 6. The image combining member 6 combines primary color images and black image with reference to the position signals to match positions of primary color images and black image to reduce positional deviation. A paper 13 for recording is transported in the direction shown by an arrow C from a paper retainer 12 to a contacting portion between the roller 9 and a transferring unit 14 through a paper path 15. In operation, the charger 42 charges the photosensitive member 1 and then, the optical system 4 expose one of primary color and achromatic color images, for example yellow, with rotation of the photosensitive member 1. One of developers 5y, 5m, 5c, and 5b, develops the electrostatic latent image. For example the developer 5y develops a yellow image. Then the yellow image is transferred to the image combining member 6 from the photosensitive member 1. Then, the charger 42 charges the photosensitive member 1 again after cleaning and the optical system 4 exposes the next image onto the photosensitive member, for example a magenta image is exposed. The magenta image is developed as similar to the yellow image and then transferred to the image combining member 6 where the yellow image has been formed. A cyan image and black image are consecutively formed and combined on the image combining member 6. The combined color image is transferred to the paper 3 by the transferring unit 14. The order of formations of primary colors and achromatic color, i.e., black is predetermined in accordance with the quality of the formed color image or the like.
A density measuring unit 43 including the density sensor 7 measures a portion of each of primary color and black images formed on the image combining member 6.
Generally, the electrophotographic, or zerography processing is subjected to the various variations of the circumstances, such as the ambient temperature, humidity, or the like, so that difference in density or hue is developed in the resultant output image though the same picture image is formed. The graph shown in FIG. 5 represents a general relation between an input image density and an output image density. This relation is referred to as a .gamma. characteristic. Each of primary color images and the achromatic image has each .gamma. characteristic, so that the difference in density or hue is developed by the variation of the circumstance.
Therefore, it is possible to obtain a more stable output picture images by compensating the .gamma. characteristics of respective primary color and achromatic color images against the variation of the circumstance. The density measuring unit 43 is provided for detecting the density of a portion of the image combining member 6 in order to obtain the .gamma. characteristics and to compensate the quality of the resultant output picture image.
A control unit (not shown) including a microprocessor generates mono color and achromatic gray scale signals in response to position signals from the position sensor 11 and expose the photosensitive member 1 to an scanning light beam 44 to form gray scales of primary colors and achromatic color on the image combining member 6 and detects or samples a density signal of the density measuring unit 43 to measuring the density of the formed gray scales in response to the position signals from the position sensor 11.
As shown in FIG. 6, the density measuring unit 43 comprises a reference voltage generator 16 for generating a reference voltage Vr, a voltage-to-current converter 17 for converting the reference voltage to a reference current If, a light emitting portion 71 of the density sensor 7 for emitting an illumination light 7a to a portion of the combining member 6, a photosensitive portion 72 of the density sensor 7 for receiving a light 7b reflected at the image combining member 6 and producing a current signal Is in accordance with an intensity of the received light, a current-to-voltage converter 18 for converting the current signal Is to a voltage signal Vs, and an amplifier 19 for outputting a density signal Vo.
The reference voltage generator 16 generates the reference voltage Vr which determines the intensity of the reference current If for controlling a brightness of the illumination light 7a. The voltage-to-current converter 17 converts the reference voltage to a reference current If. The density sensor 7 has the light emitting portion 71 and the photo-sensitive portion 72 in one. The light emitting portion 71 emits the illumination light 7a to a desired portion of the image combining member 6. The image combining member 6 moves in the direction shown by an arrow D, so that when the illumination light 7a hits a shadow portion 6a of a gray scale, the photosensitive portion 72 receives the light reflected by the shadow portion 6a of the gray scale. When the illumination light 7a hits a bare portion of the combining member 6, the photosensitive portion 72 receives the light reflected at the image combining member 6. When the other intermediate density portion of the gray scale is illuminated, a portion of the illumination light is reflected by the colorant and the other portion of the illumination light is reflected by the surface of the image combining member 6. Therefore, the photosensitive portion 72 of the density sensor 7 detects the reflective density at the image combining member 6 by representing the current signal Is. The current-to-voltage converter 18 converts the current signal Is to a voltage signal Vs. The amplifier 19 outputs a density signal Vo to the control unit (not shown) to detect the .gamma. characteristics.
FIG. 7 shows the schematic diagram of the prior art density measuring unit 43. The reference voltage generator 16 comprises resistors 20 and 21 for dividing the supply voltage Vd to generate the reference voltage Vr. The voltage-to-current converter 17 comprises resistors 22, 23, 25, and 27, an operational amplifier 24, and a transistor 26 and determines the current signal If which is given by If=Vr/resistance of resistor 27. The current signal If determines the intensity of the illumination light 7a.
The current-to-voltage converter 18 comprises resistors 28 and 29 outputs the voltage signal Vs given by Vs=Is.times.a resistance of the resistor 28. The amplifier 19 comprises resistors 30 and 31, and an operational amplifier 32 and outputs the density signal given by Vo=(1+resistance of the resistor 31/resistance of the resistor 30).times.Vs.
The reference voltage Vr and the reflectivity of the image combining member 6 is determined as follows:
FIG. 8 shows the relation of a prior art between the density of input image signal (data) and the voltage of the density signal Vo. The colorants of primary color images shows a characteristic curve 8a increasing from a reference point Vor with the density of input image signal, on the other hand the colorant of achromatic color shows a characteristic curve 8b decreasing from the reference point Vor with the density of input image signal. The reference point Vor represents the reflectivity of the boar portion of the image combining member 6. That is, the colorants of the primary colors show higher reflectivities than the combining member 6 with respective to the wavelength of the illumination light 7a. On the other hand, the colorants of the achromatic color shows a lower reflectivity than the image combining member 6. Therefore, the reference voltage Vr is determined such that the reference point Vor is positioned middle of the range of the characteristic curves 8a and 8b. That is, the intensity of the illumination light 7a is determined by the reference voltage Vr in consider of the ranges of the characteristic curves 8a and 8b.
However, in the prior art image formation apparatus mentioned above, there is a problem that dynamic ranges of the characteristic curves 8a of the primary colors and a dynamic rage of the achromatic color is smaller than the case that densities of primary colors and the density of achromatic color would be detected by different circuits. Therefore, the a resolution of density is low, so that fine density control was impossible and the picture quality was not suitably improved.